test(worker): document why TestCount is skipped

TestCount is t.Skip()'d on the branch, but the reason wasn't
recorded. Investigation: the test launches concurrent transactions
sharing entity uids and bypasses the Oracle's conflict-checking
commit path — it just calls CommitToDisk() directly with disjoint
commit timestamps. Both the legacy AddMutationWithIndex path and
the new mutation pipeline fail it identically: with two threads
adding edges to the same subject's [uid] @count predicate, neither
path can serialize @count updates without real txn conflicts, so
the count index ends up inconsistent and many subjects are missing
from count(N).

This is expected without conflict checking — the unit harness can't
exercise the safety the Oracle provides. Re-enable when we wire
either: (a) Oracle.WaitForTs/IsAborted into the harness, or (b)
this test through worker.applyMutations() (which does invoke the
Oracle conflict path).

Single-thread TestCount passes, so the per-predicate pipeline's
own count logic is correct in the absence of contention. The
existing TestStringIndexWithLang covers the multithreaded happy
path with disjoint uids.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: matthewmcneely

worker/sort_test.go

@@ -301,7 +301,8 @@ func TestStringIndexWithLang(t *testing.T) {
 }
 
 func TestCount(t *testing.T) {
-	t.Skip()
+	t.Skip("Inherently racy: bypasses the Oracle conflict-checking commit path. " +
+		"Legacy and new pipeline both fail. Re-enable when the harness uses real txn conflicts.")
 	// Setup temporary directory for Badger DB
 	dir, err := os.MkdirTemp("", "storetest_")
 	require.NoError(t, err)